Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique widely used in radiology to visualize the structure and function of a human body or a living object. Because MRI scanner is able to create excellent images of various kinds of tissues of the body, especially the soft tissue due to the water content differences in those tissues, it provides doctors useful diagnostic information pertaining all parts of the body such as nervous system, musculoskeletal system, cardiovascular system, digestive system, and urinary reproductive system. This technique is particularly advantageous for early detection and treatment of stroke, real-time observation of cardiovascular function, and diagnosis of tumors and cancer. In addition, it is an exceptional tool for orthopedic injury diagnosis and surgery for man and animal alike. It can also perform in vivo chemical analysis through spectroscopy.
During an examination, a patient's body is placed within the examination region and is supported by a patient support in an MRI scanner where a substantially constant and uniform primary magnetic field is provided by a primary magnet. The magnetic field aligns the nuclear magnetization of precessing hydrogen atoms (protons) in water in the body. Typically, there is a radio frequency (RF) coil and a gradient coil assembly within the magnet. The radio frequency coil produces an excitation frequency pulse that temporarily creates an oscillating transverse magnetization which is detected by the radio frequency coil and used by a computer system to create an image of the part of the body under examination.
To map the body precisely, magnetic field gradients is applied so that the magnitude of the magnetic field varies with location inside the examination region characteristics of the magnetic resonance signals from different locations within the region, such as the frequency and phase of the signals, can be made to vary in a predictable manner depending upon position within the region.
People have been pursuing better quality MRI imaging relentlessly since the inception of MRI in 1977. Similarly, imaging speed to minimize imaging blurring caused by patient movement or nature movement such as blood flow during imaging process is also improved. Several factors contribute to better MRI image quality in terms of high contrast and resolution. One of the critical parameter, signal-to-noise ratio (SNR), determines the image quality. Increasing SNR by increasing the signal before the pre-amplifier of the MRI system is important in terms of increasing the quality of the image. SNR is defined as the power ratio of signal and noise.SNR=Psignal/Pnoise  (Equation 1)wherein Psignal and Pnoise are the power of signal and noise, respectively.
In an MRI system, RF coils function as transmitters to apply magnetic field pulses that excite the nuclear (proton) spins. Meanwhile, the coils function as receivers to receive the weak free induction signal. The transmitter design is not nearly as crucial as the receiver design for achieving good image quality. In some cases, the same coil serves as both the transmitter and receiver to avoid the electronic switching between transmit and receive modes of operation (M. A. Foster, J. M. S. Hutchison, Practical NMR Imaging IRL Press Ltd. p 34, 1987). However, criteria of an ideal transmission coil conflict with those of the receiver coil. Therefore in most situations separate transmission coils and receiver coils are used in an MRI system.
One way to improve SNR is to increase the strength of the magnet as the SNR is proportional to the magnitude of the magnetic field. The magnitude of magnetic field in MRI is usually measured in the unit of tesla (T), of which 1 tesla equals 10,000 gauss. FDA (Food and Drug Administration) has limited the magnitude of magnetic field of MRI scanner to less than 4 tesla for use in medical imaging (FDA Guidelines for Magnetic Resonance Equipment Safety”, Center for Devices and Raiological Health, FDA, 2002).
Another way to improve SNR is to reduce noises. Since there is an upper limitation of the strength of magnetic field for human MRI, it is more sensible to reduce noise.
Noises can be categorized to body (sample) noise created by patient's body and coil noise created by the coils.Pnoise=Psignal-noise+Pcoil-noise  (Equation 2)wherein Psignal-noise and Pcoil-noise are sample and coil noise, respectively. Using (1) and (2), one can obtainSNR=Psignal/(Psignal-noise+Pcoil-noise)  (Equation 3)
In equation 3, the signal is inversely proportional to the square of the distance between the coil and the sample; the sample noise is proportional to the volume of the sample (body) or field of view (FOV). The larger the sample is, the higher the sample noise will be. On the other hand, coil noise is a function of conductance of the coil, which is determined by materials of the coil and the temperature of the coil. (Neil Alford: “Superconducting Receive Coils for a Compact Low Field MRI System”, in: Physical Electronics and Materials, http://eccel.lsbu.ac.uk/research/pem/MRI.html).
A lot of research work has been done on reducing both sample (body) noise and coil noise to improve MRI image quality. However, current art of general purpose MRI scanners have intrinsic shortcomings. A general purpose scanner has large magnets and large coils in order to scan the whole body of a patient. The large magnets provide large coverage of scanning area which is suitable for all kinds of tissue in all parts of the body. Large coils have large FOVs and create high sample noise and coil noise. In order to shorten the distance from coil to the patient's body and increase the signal strength, the coils are positioned around the patient closely, making the examination region of the scanner wrapping the patient tightly, which is often the reason of claustrophobia complaint from the patient. Most of the time, doctors are interested in examining certain organ in certain part of the patient's body using different types of the coils such as general purpose surface coil, knee coil, breast coil, head coil, spine coil, and array coil. The coils can be designed to fit to the shapes of the organs to be examined, and are positioned much closer to the organs so that the signal strength will be increased. All of the current RF coils are made of metallic material such as copper. Coils are detachable from the magnet and placed into the magnet one coil at a time to image individual organ.
A high SNR in clinic MRI imaging could be achieved by design and making better RF coils. Previous work done (“HTS Volume Coil with Improved Imaging Volume”, S. Y. Chong, ISMRM, 2008) by research groups has demonstrated that SNR can be increased by as much as 300-500% using the superconductor materials, particularly the high temperature superconductor (HTS) such as YiBaCuO, BiSrTiCaO, et al. The drawback of the superconductor coil is that one has to use a bulky cryogenic subsystem in order to operate the superconductor coil. So far, all the HTS coils demonstrated are also detached from the MRI machine. In clinical applications, the superconducting coil subsystem has to be installed and detached back or forth for imaging of individual organ. With the bulky cryogenic subsystem, it is very troublesome to do the installation which limits the application. Many old designs for the general purpose scanner such as coils, cryogenic (cooling) system for the coils, the housing for the coils and the patient support are no longer suitable for the new, high performance MRI system.